Merge branch '3.1'

* 3.1:
  [VarDumper] Fix dumping jsons casted as arrays
  PassConfig::getMergePass is not an array
  Revert "bug #19114 [HttpKernel] Dont close the reponse stream in debug (nicolas-grekas)"
  [Serializer] Include the format in the cache key
  Fix the retrieval of the last username when using forwarding
  [Yaml] Fix PHPDoc of the Yaml class
  [HttpFoundation] Add OPTIONS and TRACE to the list of safe methods
  Update getAbsoluteUri() for query string uris

Conflicts:
	src/Symfony/Component/DependencyInjection/Compiler/PassConfig.php
	src/Symfony/Component/HttpFoundation/Tests/RequestTest.php

Author: nicolas-grekas

Authentication/AuthenticationUtils.php

@@ -65,7 +65,13 @@ public function getLastAuthenticationError($clearSession = true)
      */
     public function getLastUsername()
     {
-        $session = $this->getRequest()->getSession();
+        $request = $this->getRequest();
+
+        if ($request->attributes->has(Security::LAST_USERNAME)) {
+            return $request->attributes->get(Security::LAST_USERNAME);
+        }
+
+        $session = $request->getSession();
 
         return null === $session ? '' : $session->get(Security::LAST_USERNAME);
     }